fix(IoT): Fixing a potential race condition in the timer ring queue

[ruisebas]

Issue #, if available:

Description of changes:
AWSMQTTSession.timerRing is a NSArray with each element being a NSSet. This can potentially cause a race condition that leads to a crash, if two different threads attempt to access/modify these collections at the same time.

I'm creating a new AWSMQTTTimerRing class that encapsulates these collections and protects their access with a NSLock, similar to what I previously did for AWSIoTAtomicDictionary

Check points:

• Added new tests to cover change, if needed
• All unit tests pass
• All integration tests pass
• Updated CHANGELOG.md
• Documentation update for the change if required
• PR title conforms to conventional commit style

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.